HUMUS SILICA ALUMINA. 97 



afterwards 27'0 c.c. of decinormal sodium hydrate solution. 

 Hence 50 27*0 = 23 c.c. of the acid must have been neutralised 

 by the ammonia. 1 c.c. of decinormal sulphuric acid contains 

 0049 gramme of real H. 2 S0 4 , equivalent to neutralising -0017 

 gramme of ammonia, corresponding to -0014 gramme of nitro- 

 gen. Hence the 27 c.c. of sulphuric acid correspond to 

 23 x '0014 gramme of nitrogen, and this was present in 10'868 

 grammes of soil. 



The percentage of nitrogen is therefore 

 23 x -0014x100 



-10*68- =0- 2 70% nitrogen. 



Direct estimation of humus. A method originally proposed by 

 Grandeau" is based upon the solubility of humus in ammonia. 

 About 10 grammes of the soil are treated with dilute hydro- 

 chloric acid (containing about 1 % real acid) until all lime and 

 magnesia are removed. Then the acid is washed out by water 

 and the soil residue is treated with about 15 c.c. of ammonia 

 (strong ammonia diluted with about its own volume of water) 

 for three or four hours ; the whole is then filtered and the 

 residue washed once or twice with dilute ammonia. The 

 dark-coloured solution is then evaporated in a weighed plati- 

 num dish, dried at 100, and weighed. The contents of the 

 dish are then ignited and the organic matter thus oxidised. 

 The loss of weight on ignition is the amount of humus. The 

 phosphoric acid in the residue may be determined in the usual 

 way, and, according to Grandeau, affords a good measure of the 

 available phosphoric acid of the soil. 



Determination of the silica, alumina, ferric oxide and total 

 potash. 3 or 4 grammes of the finely divided soil are weighed 

 out accurately into a conical flask, 20 c.c. of strong hydro- 

 chloric acid are added, and the whole boiled on a sand tray for 

 10 minutes, a watch glass being placed on the neck of the flask 

 to prevent loss by spirting; the flask is then placed on the 

 steam bath and digested at 100 C. for 48 hours. The liquid 

 is then diluted and filtered, the residue washed with hot water, 

 dried, ignited, and weighed. 



This is reported as silica, though in many cases it doubtless 

 still contains some refractory silicates. The filtrate and 



* r. "Analyse ties Matieres Agricoles," 18H7, Vol. I., 141. 



